scholarly journals SHMIP The subglacial hydrology model intercomparison Project

2018 ◽  
Vol 64 (248) ◽  
pp. 897-916 ◽  
Author(s):  
BASILE DE FLEURIAN ◽  
MAURO A. WERDER ◽  
SEBASTIAN BEYER ◽  
DOUGLAS J. BRINKERHOFF ◽  
IAN DELANEY ◽  
...  
Keyword(s):  
Water Pressure ◽  
Drainage System ◽  
Model Intercomparison ◽  
Ice Dynamics ◽  
Lumped Model ◽  
Annual Variations ◽  
Hydrology Model ◽  
Basal Sliding ◽  
Complex Models ◽  
Intercomparison Project

ABSTRACTSubglacial hydrology plays a key role in many glaciological processes, including ice dynamics via the modulation of basal sliding. Owing to the lack of an overarching theory, however, a variety of model approximations exist to represent the subglacial drainage system. The Subglacial Hydrology Model Intercomparison Project (SHMIP) provides a set of synthetic experiments to compare existing and future models. We present the results from 13 participating models with a focus on effective pressure and discharge. For many applications (e.g. steady states and annual variations, low input scenarios) a simple model, such as an inefficient-system-only model, a flowline or lumped model, or a porous-layer model provides results comparable to those of more complex models. However, when studying short term (e.g. diurnal) variations of the water pressure, the use of a two-dimensional model incorporating physical representations of both efficient and inefficient drainage systems yields results that are significantly different from those of simpler models and should be preferentially applied. The results also emphasise the role of water storage in the response of water pressure to transient recharge. Finally, we find that the localisation of moulins has a limited impact except in regions of sparse moulin density.

scholarly journals The impact of jökulhlaups on basal sliding observed by SAR interferometry on Vatnajökull, Iceland

2007 ◽  
Vol 53 (181) ◽  
pp. 232-240 ◽  
Author(s):  
Eyjólfur Magnússon ◽  
Helmut Rott ◽  
Helgi Björnsson ◽  
Finnur Pálsson
Keyword(s):  
Initial Phase ◽  
Water Pressure ◽  
Drainage System ◽  
Sar Interferometry ◽  
Ice Dynamics ◽  
Velocity Increase ◽  
Basal Sliding ◽  
Tunnel System ◽  
The Impact ◽  
Local Uplift

AbstractWe have analyzed InSAR data from the ERS-1/ERS-2 tandem mission, to study the ice dynamics of Vatnajökull, Iceland, during jökulhlaups from the Skaftá cauldrons and the Grímsvötn geothermal area, which drained under the Tungnaárjökull and Skeiðarárjökull outlets, respectively. During the initial phase of a Grímsvötn jökulhlaup in March 1996, the velocity of Skeiðarárjökull increased up to three-fold (relative to observed velocities in December 1995) over an area up to 8 km wide around the subglacial flood path. Accumulation of water was observed at one location in the flood path. During a small jökulhlaup from the Skaftá cauldrons in October 1995 the velocity on Tungnaárjökull increased up to four-fold over a 9 km wide area. The velocity increase was observed 1.5 days before the floodwater was detected in the river Skaftá. A reduced glacier speed as the flood peaked in Skaftá indicates evolution of the subglacial drainage system from sheet to tunnel flow. The glacier acceleration and local uplift, observed in the early phase of both jökulhlaups, supports the concept that increased water inflow in a narrow tunnel system causes water pressure to rise and forces water into areas outside the channels, thus reducing the coupling of ice with the glacier bed.

The meltwater feedbacks on ice dynamics, elevation versus lubrication

2020 ◽  
Author(s):  
Basile de Fleurian ◽  
Petra Langebroek ◽  
Paul Halas
Keyword(s):  
Water Pressure ◽  
Ice Sheet ◽  
Drainage System ◽  
Greenland Ice Sheet ◽  
Ice Dynamics ◽  
Ice Surface ◽  
Basal Sliding ◽  
Surface Melt ◽  
The One ◽  
Different Response

<p>In recent years, temperatures over the Greenland ice sheet have been rising leading to an increase in surface melt.  Projections show that this augmentation of surface melt will continue in the future and spread to higher elevations. As it increases, melt leads to two different feedbacks on the dynamic of the Greenland ice sheet. This augmentation of melt lowers the ice surface and changes its overall geometry hence impacting the ice dynamics through ice deformation. The other feedback comes into play at the base of glaciers. Here, the increase of water availability will impact the distribution of water pressure at the base of glaciers and hence their sliding velocity. The first feedback is relatively well known and relies on our knowledge of the rheology and deformation of ice. The lubrication feedback acting at the bed of glaciers is however highly uncertain on time scales longer than a season. Here we apply the  Ice  Sheet  System  Model  (ISSM)  to  a  synthetic  glacier  which  geometry  is  similar to the one of a Greenland ice sheet land terminating glacier. The dynamic contributions from ice deformation and sliding are separated to study their relative evolution. This is permitted by the use of a dynamical subglacial hydrology model that allows to link the basal sliding to the meltwater production through an appropriate friction law. The  model  is  forced  through  a  simple  temperature  distribution  and  a  Positive  Degree  Day  model which allows to apply a large range of different forcing scenarios. Of particular interest is the evolution of the distribution of the efficient and inefficient component of the subglacial drainage system and their different response to the distribution of melt during the year which directly impact the sliding regime at the base of the glacier.</p>

Greenland land-terminating glaciers velocity trends during the last two decades

2021 ◽  
Author(s):  
Paul Halas ◽  
Jeremie Mouginot ◽  
Basile de Fleurian ◽  
Petra Langebroek
Keyword(s):  
Water Pressure ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Surface Melting ◽  
Limited Area ◽  
Ice Dynamics ◽  
Basal Sliding ◽  
Surface Melt ◽  
Ice Sheet Dynamics ◽  
The Impact

<div> <p>Ice losses from the Greenland Ice Sheet have been increasing in the last two decades, leading to a larger contribution to the global sea level rise. Roughly 40% of the contribution comes from ice-sheet dynamics, mainly regulated by basal sliding. The sliding component of glaciers has been observed to be strongly related to surface melting, as water can eventually reach the bed and impact the subglacial water pressure, affecting the basal sliding.  </p> </div><div> <p>The link between ice velocities and surface melt on multi-annual time scale is still not totally understood even though it is of major importance with expected increasing surface melting. Several studies showed some correlation between an increase in surface melt and a slowdown in velocities, but there is no consensus on those trends. Moreover those investigations only presented results in a limited area over Southwest Greenland.  </p> </div><div> <p>Here we present the ice motion over many land-terminating glaciers on the Greenland Ice Sheet for the period 2000 - 2020. This type of glacier is ideal for studying processes at the interface between the bed and the ice since they are exempted from interactions with the sea while still being relevant for all glaciers since they share the same basal friction laws. The velocity data was obtained using optical Landsat 7 & 8 imagery and feature-tracking algorithm. We attached importance keeping the starting date of our image pairs similar, and avoided stacking pairs starting before and after melt seasons, resulting in multiple velocity products for each year.  </p> </div><div> <p>Our results show similar velocity trends for previously studied areas with a slowdown until 2012 followed by an acceleration. This trend however does not seem to be observed on the whole ice sheet and is probably specific to this region’s climate forcing. </p> </div><div> <p>Moreover comparison between ice velocities from different parts of Greenland allows us to observe the impact of different climatic trends on ice dynamics.</p> </div>

The meltwater feedbacks on ice dynamics, influence of melt amplitude, duration and extent.

2021 ◽  
Author(s):  
Basile de Fleurian ◽  
Petra M. Langebroeke ◽  
Richard Davy
Keyword(s):  
Water Pressure ◽  
Ice Sheet ◽  
Drainage System ◽  
Greenland Ice Sheet ◽  
System Model ◽  
Ice Dynamics ◽  
Mountain Glaciers ◽  
Different Characteristics ◽  
Crucial Parameter ◽  
Subglacial Drainage

<p>In recent years, temperatures over the Greenland ice sheet have been rising, leading to an increase in surface melt. This increase however can not be reduced to a simple number. Throughout the recent years we have seen some extreme melt seasons with melt extending over the whole surface of the ice sheet (2012) or melt seasons of lower amplitudes but with a longer duration (2010). The effect of those variations on the subglacial system and hence on ice dynamic are poorly understood and are still mainly deduced from studies based on mountain glaciers.</p><p>Here we apply the Ice-sheet and Sea-level System Model (ISSM) to a synthetic glacier with a geometry similar to a Greenland ice sheet land terminating glacier. The forcing is designed such that it allows to investigate different characteristics of the melt season: its length, intensity or the spatial extension of the melt. Subglacial hydrology and ice dynamics are coupled within ISSM is coupled to a subglacial hydrology model, allowing to study the response of the system in terms of subglacial water pressure and the final impact on ice dynamics. Of particular interest is the evolution of the distribution of the efficient and inefficient component of the subglacial drainage system which directly impacts the water pressure evolution at the base of the glacier.</p><p>We note that the initiation of the melt season and the intensity of the melt at this period is a crucial parameter when studying the dynamic response of the glacier to different melt season characteristics. From those results, we can infer a more precise evolution of the dynamics of land terminating glaciers that are heavily driven by their subglacial drainage system. We also highlight which changes in the melt season pattern would be the most damageable for glacier stability in the future.</p>

scholarly journals Investigating changes in basal conditions of Variegated Glacier prior to and during its 1982–1983 surge

2011 ◽  
Vol 5 (3) ◽  
pp. 659-672 ◽  
Author(s):  
M. Jay-Allemand ◽  
F. Gillet-Chaulet ◽  
O. Gagliardini ◽  
M. Nodet
Keyword(s):  
Flow Line ◽  
Stokes Problem ◽  
Water Pressure ◽  
Drainage System ◽  
Surface Velocity ◽  
Parameter Distribution ◽  
Basal Friction ◽  
Friction Parameter ◽  
Basal Sliding ◽  
Regular Increase

Abstract. Variegated Glacier (Alaska) is known to surge periodically after a sufficient amount of cumulative mass balance is reached, but this observation is difficult to link with changes in the basal conditions. Here, using a 10-yr dataset, consisting of surface topography and surface velocity observations along a flow line for 25 dates, we have reconstructed the evolution of the basal conditions prior to and during the 1982–1983 surge. The model solves the full-Stokes problem along the central flow line using the finite element method. For the 25 dates of the dataset, the basal friction parameter distribution is inferred using the inverse method proposed by Arthern and Gudmundsson (2010). This method is here slightly modified by incorporating a regularisation term in the cost function to avoid short wavelength changes in the friction parameter. Our results indicate that dramatic changes in the basal conditions occurred between 1973 to 1983. Prior to the surge, periodic changes can be observed between winter and summer, with a regular increase of the sliding from 1973 to 1982. During the surge, the basal friction decreased dramatically and an area of very low friction moved from the upper part of the glacier to its terminus. Using a more complex friction law, these changes in basal sliding are then interpreted in terms of basal water pressure. Our results support that dramatic changes took place in the subglacial drainage system of Variegated Glacier, moving from a relatively efficient drainage system prior to the surge to an inefficient one during the surge. By reconstructing the water pressure evolution at the base of the glacier it is possible to propose a scenario for the hydrological history leading to the occurrence of a surge.

scholarly journals Reduced glacier sliding caused by persistent drainage from a subglacial lake

2009 ◽  
Vol 3 (2) ◽  
pp. 561-578
Author(s):  
E. Magnússon ◽  
H. Björnson ◽  
H. Rott ◽  
F. Pálsson
Keyword(s):  
Steady State ◽  
Theoretical Prediction ◽  
Water Pressure ◽  
Drainage System ◽  
Pressure Reduction ◽  
Subglacial Lake ◽  
Basal Sliding ◽  
Water Accumulation ◽  
Interferometric Sar ◽  
Glacier Motion

Abstract. We present velocity observations of a glacier outlet in Vatnajökull, Iceland, deduced from interferometric SAR (InSAR) data obtained during the ERS1/2 tandem mission in 1995–2000. More than 50% decrease in glacier motion was observed subsequent to a large jökulhlaup from the subglacial lake Grímsvötn in 1996. The glacier had not reached its former flow rate in 2000. The jökulhlaup damaged the lake's ice-dam causing persistent drainage from the lake. InSAR based studies of water accumulation within Grímsvötn suggest that a leakage of >3 m3 s−1 prevailed throughout our study period. We suggest that the lake leakage kept open a tunnel at low water pressure underneath the whole length of the glacier. The tunnel flow drained water from its surroundings, hence lowering the water pressure of a distributed drainage system, underneath the upper and centre part of the glacier, which prior to the jökulhlaup sustained significant basal sliding. This is in accordance with theoretical prediction that tunnel flow in a steady state may cause slow-down in glacier motion by reducing the subglacial water pressure. The width of the affected areas was ~5 km on the upper part of the glacier and ~8 km on the centre part of the glacier. This indicates that the water pressure reduction propagates laterally from the tunnel over a distance of a few km.

scholarly journals Insensitivity of mass loss of Icelandic Vatnajökull ice cap to solar geoengineering

2021 ◽  
Author(s):  
Chao Yue ◽  
Louise Steffensen Schmidt ◽  
Liyun Zhao ◽  
Michael Wolovick ◽  
John C. Moore
Keyword(s):  
Heat Flow ◽  
Mass Loss ◽  
Mass Balance ◽  
Stratospheric Aerosol ◽  
Model Intercomparison ◽  
Geothermal Heat ◽  
Ice Dynamics ◽  
Local Climate ◽  
Ice Cap ◽  
Intercomparison Project

Abstract. Geoengineering by stratospheric aerosol injection (SAI) may reduce the mass loss from Vatnajökull ice cap (VIC), Iceland, by slowing surface temperature rise, despite relative increases in ocean heat flux brought by the Atlantic Meridional Circulation (AMOC). Although surface mass balance (SMB) is affected by the local climate, the sea level contribution is also dependent on ice dynamics. We use the Parallel Ice Sheet Model (PISM) to estimate the VIC mass balance under the CMIP5 (Coupled Model Intercomparison Project Phase 5) RCP4.5, 8.5 and GeoMIP (Geoengineering Model Intercomparison Project) G4 SAI scenarios during the period 1982–2089. The G4 scenario is based on the RCP4.5, but with additional 5 Tg yr−1 of SO2 injection to the lower stratosphere. By 2089, G4 reduces VIC mass loss from 16 % lost under RCP4.5, to 12 %. Ice dynamics are important for ice cap loss rates, increasing mass loss for RCP4.5 and G4 by 1/4 to 1/3 compared with excluding ice dynamics, but making no difference to mass loss difference under the scenarios. We find that VIC dynamics are remarkably insensitive to climate forcing partly because of AMOC compensation to SMB and low rates of iceberg calving making ocean forcing close to negligible. But the exceptionally high geothermal heat flow under parts of the ice cap which produces correspondingly high basal melt rates means that surface forcing changes are relatively less important than for glaciers with lower geothermal heat flow.

A coupled model of rapidly-evolving subglacial hydrology with ice dynamics

2021 ◽  
Author(s):  
Michael McPhail ◽  
Ian Hewitt
Keyword(s):  
Large Scale ◽  
Water Pressure ◽  
Ice Sheet ◽  
Drainage System ◽  
Momentum Equation ◽  
Effective Pressure ◽  
Short Term ◽  
Ice Dynamics ◽  
The Impact

<p>The presence of subglacial water can have a significant effect on the motion of an ice sheet. The rate at which the ice slides over the bedrock is mediated by subglacial water pressure. Meltwater on the surface of the sheet can drain through cracks and moulins; drastically increasing the amount of water under the sheet. This source of water fluctuates seasonally and diurnally, much faster than the timescale associated with large-scale glacier evolution. We are interested in the effect that this short-term variation in the subglacial hydrology, and therefore water pressure, has on the long-term behaviour of the ice sheet.<span>  </span>In particular, we are interested in how important it is to resolve the short-timescale variations in ice sliding speed.</p><p> </p><p>We use a mathematical model to study the response of the subglacial drainage system to time-varying surface melt input. By coupling this subglacial hydrology through an effective-pressure-dependent sliding law to the momentum equation for the overlying ice sheet, we study the impact of short-term meltwater fluctuations on the ice velocity.<span>  </span>We study these interactions using a one-dimensional (1D) flowline model representing a confined glacier, allowing us to explore a range of couplings between the ice flow and hydrology.<span>  </span>This enables us to assess the influence of the fluctuating meltwater input on the long-term behaviour of the ice sheet. We find that using a time-averaged effective pressure with an asynchronous coupling to the momentum equation gives a reasonable estimate for the time-averaged ice-sheet velocity, despite the nonlinearity of the governing equations. We use the results to suggest how hydrological coupling might be achieved in larger-scale models where resolving the short-term fluctuations is likely to be infeasible. <span> </span></p>

Simulating Antarctic subglacial hydrology processes underneath Pine Island Glacier, West Antarctica, using GlaDS model in Elmer/Ice

2020 ◽  
Author(s):  
yufang zhang ◽  
John Moore ◽  
Michael Wolovick ◽  
Rupert Gladstone ◽  
Thomas Zwinger ◽  
...  
Keyword(s):  
Sea Level Rise ◽  
Sea Level ◽  
Ice Sheet ◽  
Drainage System ◽  
The State ◽  
Effective Pressure ◽  
West Antarctica ◽  
Ice Dynamics ◽  
Grounding Line ◽  
Basal Sliding

<p><strong>Abstract:</strong> Very little is known about the subglacial hydrologic system under the Antarctic Ice Sheet due to the difficulty of directly observing the bottom of the ice sheet. Hydrology modeling is a powerful tool to simulate the spatial distribution of crucial hydrologic properties under the ice sheet. Here, we use the state-of-art two-dimensional Glacier Drainage System model (GlaDS) to simulate both distributed sheet flow and continuous channels under Pine Island Glacier (PIG), West Antarctica, one of the largest contributors to sea level rise in Antarctica.</p><p>We adopt an unstructured triangular mesh which enables channels to form along element edges. We drive the model with meltwater computed from an inversion and steady temperature simulation of PIG using a Stokes flow ice dynamic model. Our domain comprises the full PIG catchment. We aim to study the pattern and development of water pressure, hydraulic potential, water sheet thickness and discharge, as well as channel area and flux, which together describe the state of the basal system.</p><p>Our results for hydraulic potential correctly route water towards the grounding line, while we find near-zero effective pressure underneath the main trunk of PIG, consistent with the low basal drag and low driving stress there. This has implications for the representation of sliding in ice dynamic models: typical assumptions about hydrology connectivity to the ocean will overestimate effective pressure. When run forward in time, efficient channels evolve near the grounding line indicating an efficient drainage system where water fluxes are high in the downstream part of the PIG.</p><p>By applying GlaDS to a real marine ice sheet catchment we can better understand how basal hydrology modulates ice dynamics through basal sliding. We plan to compare our model predictions of effective pressure and drainage system with driving stress and inversions of basal drag. This will allow us to see the relationship between basal hydrology and basal sliding under PIG, and provide us better tools to predict the evolution of the region in view of future climate scenarios. Moving forward, we plan to couple the hydrology model with the ice dynamics model to make more accurate projections of sea level rise from PIG.</p><p>Key Words: West Antarctica, subglacial hydrology, drainage system, GlaDS, Elmer/Ice, Pine Island Glacier</p>

scholarly journals Seasonal and Diurnal Dynamics of Subglacial Channels: Observations Beneath an Alpine Glacier

2019 ◽  
Author(s):  
Ugo Nanni ◽  
Florent Gimbert ◽  
Christian Vincent ◽  
Dominik Gräff ◽  
Fabian Walter ◽  
...  
Keyword(s):  
Pressure Gradient ◽  
Water Discharge ◽  
Drainage System ◽  
Fold Increase ◽  
Hydraulic Pressure ◽  
Hydraulic Radius ◽  
Ice Dynamics ◽  
Alpine Glacier ◽  
Basal Sliding ◽  
Potential Link

Abstract. Water flowing below glaciers exerts a major control on glacier basal sliding speeds. However, our knowledge on the physics of subglacial hydrology and its link with sliding is limited by lacking observations. Here we use a two-year long dataset made of on-ice measured seismic and in-situ measured glacier basal sliding speed records on the Glacier d’Argentière (French Alps) to investigate the physics of subglacial channels and its potential link with glacier basal sliding. Using dedicated theory and concomitant measurements of water discharge, we quantify temporal changes in channels hydraulic radius and hydraulic pressure gradient. At seasonal timescales we observe, for the first time, that hydraulic radius and hydraulic pressure gradient present a four-fold increase from spring to summer, followed by a comparable decrease towards autumn. At low discharge during the early and late melt season channels respond to changes in discharge mainly through changes in hydraulic radius, a regime that is consistent with predictions of channels behaving at equilibrium. In contrast, at high discharge and high short-term water-supply variability (summertime), channels undergo strong changes in hydraulic pressure gradient, a behavior that is consistent with channels being out-of-equilibrium. This out-of-equilibrium regime is further supported by observations at the diurnal scale, which demonstrate that channels pressurize in the morning and depressurize in the afternoon. During summer we also observe high and sustained basal sliding speeds, supporting that the widespread inefficient drainage system (cavities) is likely pressurized concomitantly with the channel-system. We propose that pressurized channels help sustain high pressure in cavities (and therefore high glacier sliding speeds) through an efficient hydraulic connection between the two systems. Using the two regimes herein observed in channels seasonal-dynamics as constraints for subglacial hydrology/ice dynamics models may allow to strengthen our knowledge on the physics of subglacial processes.

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